1. Field of the Invention
The present invention relates to a laser drive device and an image forming apparatus incorporating the same, and more particularly to a laser drive device for use in an image forming apparatus that forms an image by scanning a laser beam modulated by image data on a photosensitive member, and an image forming apparatus incorporating the same.
2. Description of the Related Art
A laser drive device employed in an image forming apparatus, such as a copying machine, a printer, or the like, is known e.g. from Japanese Patent No. 3332916. FIGS. 4A and 4B show the arrangement of a laser drive device of this type. In FIGS. 4A and 4B, reference numeral 101 designates an image processor LSI, 102 a laser driver IC, 103 a 5V output regulator, and 104 a semiconductor laser section that is comprised of a laser diode LD and a photo diode PD for detecting the amount of light emitted from the laser diode LD. A pulse signal generated based on image data by the image processor LSI 101, and so forth are input to the laser driver IC 102.
The image processor LSI 101 is comprised of an interface controller 105, a pulse generator 106, and a laser control signal generator 107. The interface controller 105 operates in response to an image generation start signal input from an external device, to return an image request signal to the external device in predetermined timing determined by counting pulses of a main scanning synchronization signal. The pulse generator 106 operates in response to the image request signal, to generate a PWM signal (pulse width modulation signal) based on image data sent from the external device, and output the PWM signal to the laser driver IC 102. The laser control signal generator 107 counts pulses of a system clock with reference to the main scanning synchronization signal, thereby generating an APC signal for specifying a light amount control period over which is controlled the amount of light emitted from the laser diode LD, and a sample hold signal for specifying a time period over which is formed a feedback loop for the light amount control, and delivers the APC signal and the sample hold signal to the laser driver IC 102.
The laser driver IC 102 is comprised of an OR gate 108, a driving current ON/OFF control circuit 109, a driving current source 110, a reference voltage source 111, a comparator 112, and a sample hold control circuit 113, and has a current/voltage converting variable resistor 114 and a holding capacitor 115 that are externally attached thereto.
The OR gate 108 outputs the logical sum of the PWM signal from the pulse generator 106 and the APC signal from the laser control signal generator 107 to the driving current ON/OFF control circuit 109. The driving current ON/OFF control circuit 109 is turned on when the output signal from the OR gate 108 goes high, and turned off when the output signal goes low. Further, when the driving current ON/OFF control circuit 109 is on, the laser diode LD of the semiconductor laser section 104 is lighted by power supplied from the driving current source 110, and when the circuit 109 is off, the laser diode LD is extinguished.
The sample hold control circuit 113 is turned on when the sample hold signal from the laser control signal generator 107 goes high. When the sample hold control circuit 113 is on, the feedback loop for the light amount control is formed, and the amount of current flowing to the laser diode LD is controlled according to the amount of light detected by the photo diode PD.
More specifically, the laser beam emitted from the laser diode LD enters the photo diode PD where the incident laser beam is photoelectrically converted. Then, the resulting electric current is converted into a voltage by the current/voltage converting variable resistor 114, and input to the comparator 112 for comparison with a reference voltage from the reference voltage source 111. When the voltage reflecting the light amount of the laser diode LD is higher than the reference voltage, the comparator 112 outputs a low-level output, and when the voltage is lower than the reference voltage, the comparator 112 outputs a high-level output, to thereby carry out charging and discharging of the holding capacitor 115. When the sample hold signal goes low, the sample hold control circuit 113 is turned off to maintain the voltage level of the holding capacitor 115, and the driving current source 110 determines the amount of current, that is, the amount of light (laser power) for driving the laser diode LD according to the maintained voltage level.
FIG. 5 shows the arrangement of a laser scanner for a copying machine or a printer to which is applied the laser drive device shown in FIGS. 4A and 4B. In FIG. 5, reference numeral 201 designates an image data-processing unit in which is mounted the image processor LSI 101 appearing in FIG. 4A. Reference numeral 202 designates a laser driver unit in which are mounted the laser driver IC 102, the 5V output regulator 103, and the semiconductor laser section 104, all appearing in FIG. 4B.
Reference numeral 203 designates a collimator lens for causing a laser beam irradiated from the semiconductor laser section 104 to converge into parallel light. Reference numeral 204 designates a polygon mirror which is rotated by a motor, not shown, in a direction indicated by the arrow in FIG. 5, to thereby reflect the laser beam for scanning. Reference numeral 205 designates a photosensitive drum that receives the laser beam reflected from the polygon mirror 204 to form an electrostatic latent image on a surface thereof. In doing this, the photosensitive drum 205 is irradiated with the laser beam while being rotated in the direction indicated by the arrow. Reference numeral 206 designates an fθ lens which deflects the laser beam from the polygon mirror 204 such that the scanning speed of the laser beam on the photosensitive drum 205 is made uniform from the center to each end of the drum surface.
The photosensitive drum 205 has one end thereof formed with a reference mark 207 for use in image formation. A sensor 208 is provided to detect the reference mark 207 and deliver the image generation start signal to the interface controller 105 of the image data-processing unit 201 (see FIG. 4A). Further, a sensor 209 is disposed on an extension line from the main scanning line along which the laser beam performs exposure scanning of the photosensitive drum 205. Whenever detecting the laser beam, the sensor 209 delivers the main scanning synchronization signal to the interface controller 105 and the laser control signal generator 107 of the image data-processing unit 201 (see FIG. 4A)
FIG. 6A is a timing chart macroscopically showing the relationship between the image generation start signal, the main scanning synchronization signal, and the image request signal.
The image generation start signal and the main scanning synchronization signal are delivered as described above, whereas the image request signal is delivered as follows: The interface controller 105 of the image processor LSI 101 installed in the image data-processing unit 201 counts pulses of the main scanning synchronization signal with reference to the image generation start signal, and upon the lapse of a predetermined time period T1 after receiving the image generation start signal, the interface controller 105 delivers the image request signal to the external device. The external device starts counting pulses of the main scanning synchronization signal upon reception of the image request signal, and upon the lapse of a predetermined time period T2 after the start of the counting, starts outputting image data. During a time period (T3-T2) until a predetermined time period T3 elapses after the start of the counting of pulses of the synchronization signal, image data can be outputted, that is, a valid image is present on the photosensitive drum 205.
FIG. 6B is a timing chart microscopically showing the relationship between the main scanning synchronization signal, the image data, the APC signal, and the sample hold signal. Upon reception of the main scanning synchronization signal, the laser control signal generator 107 of the image processor LSI 101 appearing in FIG. 4A delivers to the laser driver IC 102 the APC signal for specifying the light amount control period and the sample hold signal for specifying the time period over which the feedback loop for the light amount control is formed.
In the above case, as shown in FIG. 6B, the laser control signal generator 107 provides control such that when a predetermined time period T5 has elapsed after a fall of the immediately preceding pulse of the main scanning synchronization signal, the following pulse of the APC signal is caused to rise, and when a predetermined time period T6 has elapsed after a fall of the following pulse of the main scanning synchronization signal, the following pulse of the APC signal is caused to fall.
Further, the laser control signal generator 107 provides control such that each pulse of the sample hold signal occurs and lasts within the duration of a corresponding pulse of the APC signal. This prevents the sample hold signal from becoming enabled to form the feedback loop when the APC signal is in a disabled state and the laser diode LD is extinguished, since such feedback loop formation can result in excessive charging of the holding capacitor 115 and hence the amount of current to be supplied to the laser diode LD from the driving current source 110 being set to an excessively large value, causing breakage of the laser diode LD.
The external device starts outputting image data in an amount corresponding to one main scanning line when a predetermined time period T4 has elapsed with reference to the main scanning synchronization signal. This image data is converted by the pulse generator 106 of the image processor LSI 101 into a pulse signal (PWM signal: pulse width modulation signal) for causing the laser diode LD to emit light, and then is outputted to the laser driver IC 102. The laser driver IC 102 drivingly controls the laser diode LD based on the PWM signal from the pulse generator 106, to thereby form an electrostatic latent image using a laser beam modulated based on the image data.
However, the above-described laser drive device necessitates provision of the laser driver IC 102 in addition to the image processor LSI 101, and also the regulator 103 for supplying the 5V power to the laser driver IC 102, which increases the cost of the laser drive device.